1. Field of the Invention
The present invention relates to a method for ceramic crystallization and growth. More specifically, the present invention relates to a crystal growth method based upon a hybrid Stockbarger zone-leveling melting system for seeded and unseeded crystallization providing zone-melting function and more particularly for the manufacture of large-sized crystals of lead magnesium niobate-lead titanate (PMN-PT) solid solutions and related piezocrystals by doping.
2. Description of the Related Art
Acoustic transducers are the operational center of many medical and commercial imaging systems. The most common types of transducers utilize lead zirconate titanate (PZT) based ceramics as a piezoelectric function. Piezoelectric ceramics convert mechanical energy into electrical energy and conversely electrical energy into mechanical energy. While conventional PZT materials remain the most common materials used in acoustic transduction devices, changing material requirements have fostered the need for new piezoelectric materials having improved dielectric, piezoelectric and mechanical properties.
Single crystals of solid solutions of Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Zn1/3Nb2/3)O3 (PZN) with PbTiO3 (PT) (PMN-PT and PZN-PT) have generally desirable ultrahigh piezoelectric properties, coupling constants (k33), and dielectric constants that are unachievable in conventional piezoelectric (PZT) ceramics.
At ambient temperatures, the morphotropic phase boundary (MPB), separating rhombohedral phase from tetragonal phase, exists in (1−x)PMN-xPT system at about x=0.34, and in (1−x)PZN-xPT system at about x=0.09. The crystals of compositions close to the MPB, the so-called relaxor-based single crystals, have shown greatly desired piezoelectric properties suitable for use in medical imaging devices. Unfortunately, the electromechanical properties of these types of single crystals are very sensitive to the orientation and chemical composition of the crystal (See for example U.S. Pat. No. 6,465,937), and have been very hard to produce in reliable and homogenous quantities.
In early 1980s, Kuwata et al. (J. Kumata, K. Uchino and S. Nomura, Dielectric and piezoelectric properties of 0.91Pb(Zn1/3Nb2/3)O3-0.009PbTiO3, Jpn. J. Appl. Phys., 21, 1298-1302 (1982)) found very high piezoelectric coefficient, d33, of 1500 pC/N and electromechanical coupling factor, k33, of 0.92 in 0.91PZN-0.09PT single crystals along <001> direction.
Later, high properties were also observed in PMN-PT crystals by Shrout and his co-workers in 1990 (T. R. Shrout, Z. P. Chang, N. Kim and S. Markgraf, Dielectric behavior of single crystals near the (1−x) Pb(Mg1/3Nb2/3)O3-xPbTiO3 Morphotropic Phase Boundary, Ferroelectrics Lett, 12, 63-69 (1990)).
High electromechanical coupling (k33)>90%, piezoelectric coefficient (d33)>2500 pC/N and large strain up to 1.7% were reproducibly observed in the later 1990's (S. E. Park and T. R. Shrout, Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals, J. Appl. Phys., 82, 1804-11 (1997)).
The super-high piezoelectric properties noted in this literature promised a new generation of acoustic transduction devices.
The small single crystals of PMN-PT and PZN-PT discovered above were obtained by a flux growth method. Unfortunately, usefully sized single crystals (inch size) of good quality were long unavailable until in 1997 when PZN-PT single crystals were grown by improved flux growth methods. See S. E. Park and T. R. Shrout, Characteristics Of Relaxor-Based Piezoelectric Single Crystal For Ultrasonic Transducers, IEEE Trans. On Ultrasonics, Ferroelectrics and Frequency Control, Vol. 44, No. 5, 1140-1147 (1997); and T. Kobayashi, S. Shimanuki, S. Saitoh, and Y. Yamashita, Improved Growth Of Large Lead Zinc Niobate Titanate Piezoelectric Single Crystals For Medical Ultrasonic Transducers, Jpn. J. Appl. Phys., 36, 6035-38 (1997).
A Bridgman method (P. W. Bridgman, Proc. Am. Acad. Sci. 60 9 (1925) 303) is characterized by a relative translation of a crucible containing a melt along a single axial temperature gradient in a vertical furnace. A Stockbarger method (D. C. Stockbarger, Ref. Sci. Instrum. 7 (1963) 133) is a modification of the Bridgman method and employs a single heat insulation buffer separating a vertical furnace into only two zones, a high temperature zone and an upper low-temperature zone.
Recently, a modified vertical Bridgman growth method was developed for large sized crystals: PZN-PT single crystals associated with flux (Y. Hosono, K. Harada, S. Shimanuki, S. Saitoh, and Y. Yamashita, Crystal Growth And Mechanical Properties Of Pb(Zn1/3Nb2/3)O3-Pbtio3 Single Crystal Produced By Solution Bridgman Method, Jpn. J. Appl. Phys., 38, 5512-15 (1999)) and PMN-PT single crystals using a crucible moving-downward method in a broad 2-zone temperature gradient (Chinese Pat. No. CN 1227286A, “Method Of Preparation Of Relaxor Ferroelectric Single Crystal Lead Magnesium Niobate-Lead Titanate” by H. Luo et al., published Sep. 1, 1999 and H. Luo, G. Xu, H. Xu, P. Wang, and Z. Yin, Compositional Homogeneity And Electrical Properties Of Lead Magnesium Niobate Titanate Single Crystals Grown By A Modified Bridgman Technique, Jpn. J. Appl. Phys., 39, 5581-85 (2000).
Unfortunately, substantial challenges still exist in manufacturing piezoelectric single crystals. One challenge is that a lead-contained melt, at high temperature, is made highly toxic through the evaporation of lead oxide and increases compositional segregation detrimentally. This challenge alone eliminates most commercially available crystal growth techniques. Further, the electromechanical properties of the relaxor-based PMN-PT crystals with 25˜35% PT contents close to the MPB are critically sensitive to the PT content. An additional challenge is that crystal growth with flux association yields a very low growth rate and unacceptable imperfection manifestations, including micro inclusions. Finally, each of these methods provides poor homogeneity and greatly reduced material utilization factors raising production costs.
It is also clear that the Bridgman-type growth method alone is only feasible for PMN-PT crystal due to the pseudo-congruent behavior of the binary solid solution system. So far no publications gave the reason for this behavior and there is no calculable way to predict it due to the absence of most important of the thermodynamic parameters. (Only the experimental results, presented herein indicate the crystallization behavior)
Referring now to FIGS. 1(A) and 1(B), the Bridgman growth method allows for PMN-PT crystal growth at relatively fast rates, up to 1 mm/hr, but the resultant compositional segregation is detrimentally large. The PT variability provides unpredictable and undesirable piezoelectric properties reducing material utilization to a vary small range. The resultant compositional segregations prevent commercial implementation of rapid growth rates without unacceptably high quality control losses.
Referring now to FIG. 2, single PMN-PT crystals grown using solely the Bridgman growth method resulted in great Pb, Mg, Nb, and Ti variability along the length of the boule. The growth parameters were: seeding [110], growth rate 0.8 mm/hr at temperature gradient 30° C./cm, and maximum crucible temperature of 1365° C. The Induction Coupled Plasma (ICP) spectroscopy employed had accuracy greater than 0.5%. It is clear from the figure that there is wide compositional variability along the length of the boule. This variability is drastically significant even within 1 cm length increments. It is clear that this method of crystal growth is incapable of providing useful lengths of compositionally homogenous material.
As noted above, since the piezoelectric properties of PMN-PT single crystals are sharply dependent upon composition, this composition variability results in a great reduction of the useful portion of the as-grown crystal boule, and increased production, handling, and testing costs. As seen in the figure, the % of each of the compositional changes is as much as 10-25% within 1 cm. This variability is unacceptable for commercial implementation.
In summary, the problems of commercially available manufacturing methods for PMN-PT single crystals include at least the following:    1. low unit yield and high manufacturing cost    2. gross compositional inhomogeneity resulting in variation of piezoelectric properties